This invention relates to ball nut type steering gears for motor vehicles, and more particularly to a ball nut type steering gear which enables the feel of the steering wheel in the hands of the driver to be improved when a motor vehicle is driven straight ahead at high speed.
In ball nut type steering gears of the prior art, there is certain play between the balls and the helical grooves in which the balls are rollingly engaged so as to permit the balls to smoothly roll in the helical grooves during operation, by taking into consideration possible errors which might be committed during manufacture with regard to the configuration of the helical threads of the ball nut and the threaded shaft, the lead of the threads and the circularity of the helical grooves.
On the other hand, when a motor vehicle is driven at high speed, a reaction of the tires is transmitted to the driver by way of the steering gear and steering wheel. The motor vehicle fitted with a steering gear of the prior art described hereinabove would deviate from a desired course in travel, particularly when driven at high speed, thereby making the driver feel uneasy about the stability of driving. This stems from the fact that the play referred to hereinabove between the balls and the helical grooves causes the motor vehicle to travel in serpentine movement and makes it impossible to drive the vehicle in a stable manner, particularly when the motor vehicle is driven to travel along a straight course at high speed.
In order to obviate the aforesaid disadvantages of the prior art, proposal has been made to provide a major diameter portion in the middle range of the threaded shaft, and tapering portions connected at one end thereof to the major diameter portion and having a minor diameter portion at the other end thereof, so as to reduce to zero the play between the balls and the helical grooves in which the balls are rollingly engaged during the travel of the vehicle along a straight course and to increase the rigidity of the connection between the threaded shaft and the ball nut. The aforesaid diameters may be either the pitch diameter or the root diameter of the thread of the threaded shaft. Some disadvantages are associated with the system proposed. Generally, the steering gear of this type has two trains of balls, and the balls of one train would contact the major diameter portion of the threaded shaft in the middle range thereof when the steering wheel is manipulated to turn the steering shaft through a small angle of rotation. Thus there would be little or no play between the balls of one train and the helical grooves in which they are rollingly engaged, thereby requiring the exertion of a force in manipulating the steering wheel.